1. Technical Field
The invention is related to heating and cooling apparatus in an inductively coupled RF plasma reactors of the type having a reactor chamber ceiling overlying a workpiece being processed and an inductive coil antenna adjacent the ceiling.
2. Background Art
In a plasma processing chamber, and especially in a high density plasma processing chamber, RF (radio frequency) power is used to generate and maintain a plasma within the processing chamber. As disclosed in detail in the above-referenced applications, it is often necessary to control temperatures of surfaces within the process chamber, independent of time varying heat loads imposed by processing conditions, or of other time varying boundary conditions. In some cases where the window/electrode is a semiconducting material, it may be necessary to control the temperature of the window/electrode within a temperature range to obtain the proper electrical properties of the window. Namely, for the window/electrode to function simultaneously as a window and as an electrode, the electrical resistivity is a function of temperature for semiconductors, and the temperature of the window/electrode is best operated within a range of temperatures. The application of RF power to generate and maintain the plasma leads to heating of surfaces within the chamber, including windows (such as used for inductive or electromagnetic coupling of RF or microwave power) or electrodes (such as used for capacitive or electrostatic coupling of RF power, or for terminating or providing a ground or return path for such capacitive or electrostatic coupling of RF power) or for combination window/electrodes. Heating of those surfaces can occur due to 1) ion or electron bombardment, 2) absorption of light emitted from excited species, 3) absorption of power directly from the electromagnetic or electrostatic field, 4) radiation from other surfaces within the chamber, 5) conduction (typically small effect at low neutral gas pressure), 6) convection (typically small effect at low mass flow rates), 7) chemical reaction (i.e. at the surface of the window or electrode due to reaction with active species in plasma).
Depending on the process being performed with the plasma process chamber, it may be necessary to heat the window or electrode to a temperature above that temperature which the window or electrode would reach due to internal sources of heat as described above, or it may be necessary to cool the window or electrode to a temperature below that temperature which the window or electrode would reach due to internal sources of heat during some other portion of the operating process or sequence of processes. In such cases, a method for coupling heat into the window or electrode and a method for coupling heat out of the window or electrode is required.
Approaches for heating window/electrodes from outside the process chamber include the following:
1. heating the window/electrode by an external source of radiation (i.e., a lamp or radiant heater, or an inductive heat source),
2. heating the window/electrode by an external source of convection (i.e. forced fluid, heated by radiation, conduction, or convection),
3. heating the window/electrode by an external source of conduction (i.e., a resistive heater).
The foregoing heating methods, without any means for cooling, limit the temperature range available for window or electrode operation to temperatures greater than the temperature which the window or electrode would reach due to internal sources of heat alone.
Approaches for cooling window/electrodes from outside the process chamber include the following:
1. cooling the window/electrode by radiation to a colder external surface,
2. cooling the window/electrode by an external source of convection (i.e., natural or forced),
3. cooling the window/electrode by conduction to an external heat sink.
The foregoing cooling methods, without any means for heating other than internal heat sources, limit the temperature range available for window or electrode operation to temperatures less than that temperature which the window or electrode would reach due to internal sources of heat alone.
Additionally the foregoing cooling methods have the following problems:
1. cooling the window/electrode by radiation is limited to low heat transfer rates (which in many cases are insufficient for the window or electrode temperature range required and the rate of internal heating of window or electrodes) at low temperatures due to the T4 dependence of radiation power, where T is the absolute (Kelvin) temperature of the surface radiating or absorbing heat;
2. cooling the window/electrode by an external source of convection can provide large heat transfer rates by using a liquid with high thermal conductivity, and high product of density and specific heat when high flow rates are used, but liquid convection cooling has the following problems:
A) it is limited to maximum temperature of operation by vapor pressure dependence of liquid on temperature (i.e. boiling point) (unless a phase change is allowed, which has its own problemsxe2x80x94i.e. fixed temperature of phase changexe2x80x94no control range, as well safety issues),
B) incompatibility of liquid cooling with the electrical environment, depending upon liquid electrical properties,
C) general integration issues with liquid in contact with reactor structural elements. Cooling the window or electrode by an external source of convection (e.g., a cooling gas) is limited to low heat transfer rates which in many cases are insufficient for the window or electrode temperature range required and the rate of internal heating of window or electrodes;
3. cooling the window/electrode by conduction to an external heat sink can provide high rates of heat transfer if the contact resistance between the window or electrode and the heat sink is sufficiently low, but low contact resistance is difficult to attain in practice.
Approaches for both heating and cooling window/electrodes from outside the process chamber include heating the window/electrode by an external source of conduction (i.e., a resistive heater) in combination with cooling the window/electrode by conduction to an external heat sink. In one implementation, the structure is as follows: a window or electrode has a heater plate (a plate with an embedded resistive heater) adjacent an outer surface of the window electrode. Additionally, a heat sink (typically liquid cooled) is placed proximate the opposite side of the heater plate from the window or electrode. Contact resistances are present between window or electrode and heater plate, and between the heater plate and the heat sink. In such a system integrated with automatic control of window or electrode temperature, a temperature measurement is made (continuously or periodically) of the window or electrode to be controlled, the temperature measurement is compared with a set point temperature, and based on the difference between the measured and set point temperatures a controller determines through a control algorithm how much power to apply to the resistive heater, or alternatively, how much cooling to apply to the heat sink, and the controller commands output transducers to output the determined heating or cooling levels. The process is repeated (continuously or periodically) until some desired degree of convergence of the window or electrode temperature to the set point temperature has occurs, and the control system remains active ready to respond to changes in requirements of heating or cooling levels due to changes in internal heat or cooling levels or to changes in the set point temperature. Besides contact resistance problems that limit the cooling capability of the system to control the temperature of the window or electrode, the system exhibits a time lag in transferring heat from the window or electrode to the head sink as required when the internal heating or cooling load changes during plasma reactor operation. This is due in part to the contact resistance between the window or electrode and the heater, and contact resistance between the heater and the heat sink, as well as the thermal capacitance of the heater and the window or electrode. For example, as the internal heat load is increased in a process or sequence of processes, the system senses the increase by measuring an increase in window or electrode temperature. As described above, the system reduces the heater power or increases the cooling power in response to the increase in window or electrode temperature, but there is a lag time for the heat to diffuse through the window or electrode, across the contact resistance between window or electrode and heater, through the heater plate, across the contact resistance between the heater and heat sink. In addition, xe2x80x9cexcessxe2x80x9d heat energy xe2x80x9cstoredxe2x80x9d in the heater diffuses across the contact resistance between the heater and heat sink. This lag causes more difficulty in controlling the temperature of the window or electrode as the internal heat or cooling load changes, typically resulting in some oscillation of the window or electrode temperature about the set point.
A further problem for a window or window/electrode (of the type that allows electromagnetic or inductive RF or microwave power to be coupled from outside the chamber to inside the chamber via the window or window/electrode) is that the presence of heat transfer apparatus (heater and/or heat sinks) interferes with the coupling of such electromagnetic or inductive RF or microwave power, and/or the presence of RF or microwave power coupling apparatus may interfere with heat transfer between heater and/or heat sink and window or window/electrode.
Thus a method is sought for heating and/or cooling a window or electrode or window electrode used in a plasma processing chamber so that the temperature of the window or electrode or window/electrode may be controlled sufficiently close to a set point such that a desired process or sequence of processes may be carried out within the plasma process chamber, independent of the change of internal heating or cooling loads within the chamber or changes in other boundary conditions.
Additionally, a method is sought for heating and/or cooling a window or window/electrode used in a plasma processing chamber so that the temperature of the window or electrode or window/electrode may be controlled sufficiently close to a set point temperature, without interference to coupling of electromagnetic or inductive RF or microwave power through the window or window/electrode such that a desired process or sequence of processes may be carried out within the plasma process chamber, independent of the change of internal heating or cooling loads within the chamber or changes in other boundary conditions.
Additionally, a method is sought for heating and/or cooling an electrode or window/electrode used in a plasma processing chamber so that the temperature of the electrode or window/electrode may be controlled sufficiently close to a set point temperature, without interfering with capacitive or electrostatic coupling of RF power, or interfering with terminating or providing a ground or return path for such capacitive or electrostatic coupling of RF power, such that a desired process or sequence of processes may be carried out within the plasma process chamber, independent of the change of internal heating or cooling loads within the chamber or changes in other boundary conditions.
Additionally, a method is sought for heating and/or cooling a window or electrode or window/electrode used in a plasma processing chamber so that the temperature of the electrode or window/electrode may be controlled sufficiently close to a set point temperature, without interfering with capacitive or electrostatic coupling of RF power, or interfering with terminating or providing a ground or return path for such capacitive or electrostatic coupling RF power, and without interfering with coupling of electromagnetic or inductive RF or microwave power through the window or window/electrode such that a desired process or sequence of processes may be carried out within the plasma process chamber, independent of the change of internal heating or cooling loads within the chamber or changes in other boundary conditions.
The invention is embodied in a plasma reactor including a plasma reactor chamber and a workpiece support for holding a workpiece near a support plane inside the chamber during processing, the chamber having a reactor enclosure portion facing the support, a cold plate overlying the reactor enclosure portion, a plasma source power applicator between the reactor enclosure portion and the cold plate and a thermally conductor between and in contact with the cold plate and the reactor enclosure. Preferably, the power applicator includes plural radially dispersed applicator elements defining voids therebetween and the thermal conductor includes radially dispersed thermally conductive elements in the voids and contacting the cold sink and the reactor enclosure portion. The radially dispersed thermally conductive elements preferably include respective concentric cylindrical rings. The reactor enclosure portion includes a ceiling, the ceiling including a window for power emanating from the plasma source power applicator. The power applicator preferably includes an inductive antenna in communication with an RF. power generator and the ceiling is inductive power window. The ceiling preferably but not necessarily includes a semiconductor window electrode. The thermal conductor and the cold sink define a cold sink interface therebetween, the reactor preferably further includes a thermally conductive substance within the cold sink interface for reducing the thermal resistance across the cold sink interface. The thermally conductive substance can be a thermally conductive gas filling the cold plate interface. Alternatively, the thermally conductive substance can be a thermally conductive solid material. The reactor can include a gas manifold in the cold plate communicable with a source of the thermally conductive gas an inlet through the cold plate from the gas manifold and opening out to the cold plate interface. The reactor can further include an O-ring apparatus sandwiched between the cold plate and the thermal conductor and defining a gas-containing volume in the cold plate interface in communication with the inlet from the cold plate. The gas-containing volume preferably is of nearly infinitesimal thickness. The thermal conductor can be integrally formed with the reactor enclosure portion. Alternatively, the thermal conductor can be formed separately from the reactor enclosure portion whereby a reactor enclosure interface is defined between the reactor enclosure portion and the thermal conductor, in which case preferably there is a thermally conductive substance within the reactor enclosure interface for reducing the thermal resistance across the reactor enclosure interface. The thermally conductive substance in the reactor enclosure interface can be a thermally conductive gas filling the reactor enclosure interface. Alternatively, the thermally conductive substance in the reactor enclosure interface includes a thermally conductive solid material. If the thermal conductor is formed separately from the reactor enclosure portion whereby a reactor enclosure interface is defined between the reactor enclosure portion and the thermal conductor, then preferably there is a thermally conductive gas filling the reactor enclosure interface for reducing the thermal resistance across the reactor enclosure interface. In this case, preferably there is a gas channel through the thermal conductor and communicating between the cold plate interface and the reactor enclosure interface. Preferably in this case, there is further an O-ring apparatus between the reactor enclosure portion and the thermal conductor defining a gas-containing volume in the reactor enclosure interface in communication with the gas channel of the thermal conductor. Preferably, the thermally conductive solid material includes a soft metal of the type including one of aluminum, indium, copper, nickel. Alternatively, the thermally conductive solid material includes an elastomer impregnated with particles of a thermally conductive material. The particles can be of a soft metal, such as a thermally conductive material including one of aluminum, indium, copper, nickel. Alternatively, the particles can be of a high electrical resistivity and high thermal conductivity, such as boron nitride, high resistivity silicon carbide, high resistivity silicon, aluminum nitride, aluminum oxide.